This invention relates to an image reading apparatus and image reading method for scanning and optically reading a document image and outputting the electronic data. More particularly, this invention relates to an image reading apparatus equipped with two optical reading sensors for reading the images on the upper and lower surfaces of a document or sheet fed one by one from a document reception tray by an automatic document feeder commonly known as an ADF.
Image reading apparatuses, normally called scanners, optically read the image of a document and convert the image into electronic data and transmit it to image forming apparatuses, such as personal computers, copy machines or facsimiles. Image forming apparatuses are also normally equipped with the following types of image reading apparatuses.
The image reading apparatuses have image sensors for reading the images on a document or sheet transferred one by one from a document reception tray by an automatic document feeder. Furthermore, recent image reading apparatuses have been proposed to have functions to read the images on the upper and lower surfaces of a document or sheet fed one by one by the automatic document feeder on one document transport path without transferring to a switch-back pass, by using two individual image sensors.
Examples of the image reading apparatuses that are equipped with two image sensors to read the images on the upper and lower surfaces of a document are Japanese Patent Publications (KOKAI) No. 62-258552 (hereinafter referred to as prior art 1), and No. 10-145538 (hereinafter referred to as prior art 2).
The image reading apparatus as disclosed in prior art 1 discloses lighting apparatuses for both upper surface and lower surface of a document or sheet, and an optical system that employs CCD and a line sensor to simultaneously read indicia on both surfaces of the document.
Also, in the facsimile apparatus disclosed in the prior art 2, as shown in FIG. 15 (FIG. 2 and FIG. 5 of the same publication), a line image sensor 123 for reading the indicia on the upper surface of a document starts to read the image data on the upper surface based on the detection of the leading edge of the document by a document sensor 122, and a line image sensor 124 for reading the images on the bottom surface of the document starts to read the image data thereof. Also, because the line image sensor 124 is located at a position away from the line image sensor 123 for a distance of only L2, if both line image sensors are driven at the same time, the image signals on both surfaces offset the line count equivalent to the distance of L2. Thus, by always monitoring the offset line count, transport errors of the document are determined.
Also, as an image sensor reading operation controller, it employs the output signals from the image sensor and a clock for synchronization as well as a drive controller to stop reading, as the method to control the reading operation.
In order to form a lighter weight and more compact image reading apparatus for reading the image data on both surfaces of documents or sheets, it is essential to locate a reading position for reading the images on one surface of a document (upper surface) and a reading position for reading the images on the other surface of a document (lower surface) as close as possible without a large gap therebetween. If the upper surface reading position and the lower surface reading position are disclosed close to each other, there is increased a danger of the light from the light source device for illuminating one surface of the document passing through the document sheet and being detected by the image sensor on the other surface, so that the mutual interferences of the upper and lower light sources cause electronic noise in the image data read from both surfaces.
However, in the prior art 1, both the line sensor for reading the upper surface of the document and the line sensor for reading the lower surface of the document use optical system (optical reduction type employing CCD) line sensors. Therefore, if the reading position for the one or upper surface of the document and the reading position for the other or lower surface of the document are arranged in the same position, the apparatus will become more compact and lightweight, but image data read from both surfaces will cause noise to be generated.
Also, in the prior art 2, the first line sensor for reading the indicia on the upper surface of the document starts to read the image data from the upper surface of the document based upon the detection of the leading edge of the document by the document sensor, and the second line sensor for reading the indicia on the lower surface of the document operates simultaneously with the detection by the document sensor. In such case, if monitoring the line count offset according to the distance LD for the image signal of both surfaces, an unnecessary amount of image data is recorded, requiring the entire controller device to be more complex, thus making it impossible to lighten the weight of the apparatus and to make the overall apparatus more compact. Furthermore, in the prior art 2, as in the prior art 1, there is no disclosure regarding the reduction of the image noise that occurs by the mutual interferences of the upper and lower light sources in reading the image data from both surfaces.
Thus, an object of the present invention is to provide a document reading apparatus to read indicia on both surfaces of a document or sheet using two image sensors, which is light in weight and more compact, and has a small distance of the gap between a first image sensor and a second image sensor to reduce the image noise caused by the mutual interferences of the upper and lower light sources as little as possible.
Another object of the present invention is to provide a document reading apparatus to read the indicia on both surfaces of the document using two image sensors, wherein a controller for reading images is simplified to reduce the weight and to make the apparatus more compact.
Further objects and advantages of the invention will be apparent from the following description of the invention.